Field of the Invention
The present invention relates to a coated stainless steel member, and more specifically a stainless steel member having a coating excellent in adhesion and corrosion prevention. In particular, the present invention relates to a stainless steel fuel inlet pipe (FIP) for an automobile fuel tank.
Brief Description of the Related Art
In one kind of the conventionally known methods for coating a stainless steel member, strike nickel plating is performed on a stainless steel substrate. For example, several methods are known such as a method including Wood's strike nickel plating, a subsequent chromate treatment and phosphate treatment, and spray coating (Japanese Patent Application Publication No. Sho 63-192877); a method including strike nickel plating, subsequent nickel plating, and coating (Japanese Patent Application Publication No. Hei 9-268398 and Japanese Patent Application Publication No. 2005-144895), and a method including Wood's strike nickel plating, and subsequent coating by forming a thin polymer film by reduction-electrolytic polymerization (Japanese Patent Application Publication No. 2000-212799). However, in any of these methods, coating is performed on a strike nickel plating after a treatment for improving the adhesion is performed on the strike nickel plating. This is because of the conventional recognition that “a coating film easily peels off when coating is performed directly on a nickel strike plating” (in Lower Left column on Page 2 of Japanese Patent Application Publication No. Sho 63-192877).
In general, for coating a material with good adhesion, a pre-coating surface treatment such as a zinc phosphate treatment, a chromate treatment, or a zirconium treatment is carried out. When the material is made of iron or aluminum, a chemical conversion film is formed by a reaction (dissolution and deposition) between the material and a surface treatment liquid. The coating adhesion with the material is secured by hydrogen bonding or an anchor effect caused by the chemical conversion film. However, in a case of coating on stainless steel, a surface treatment film such as a zinc phosphate film formed for enhancing the coating adhesion cannot be formed, and hence a sufficient coating adhesion cannot be obtained, because a stable oxide film is formed on a surface of the stainless steel. For this reason, peeling and corrosion occur in a secondary adhesion test for hot-salt water resistance or a chipping test. As shown in Japanese Patent Application Publication No. 2006-231207, thick coating in a thickness of 150 to 400 μm has been attempted for improvement in chipping resistance, prevention of permeation of salt water, and improvement in adhesion. However, such a method is not economical.
In the Meantime, in areas heavily inflicted with salt injury due to sea-shore salt or snow melting salt spread in a large amount, stainless steel FIPs have been used instead of conventional iron FIPs. However, even in the case of stainless steel FIPs, the corrosion resistance is insufficient at welded portions and the like. Hence, electrodeposition coating as shown in Japanese Patent Application Publication No. 2002-242779 has been carried out. The coating is conducted by other methods such as cationic electrodeposition coating or hexavalent chromium-free water-soluble acrylic-based silicone coating shown in Japanese Patent Application Publication No. 2004-230419, and coating using an acrylic paint, an alkyd paint, a urethane paint, an epoxide paint, or the like in the form of a cationic aqueous solution or emulsion shown in Japanese Patent Application Publication No. 2005-206064. However, it is difficult to perform electrodeposition excellent in adhesion and corrosion prevention on a stainless steel substrate, as described above.